Against the Tide of Years — S.M. Stirling

In the years since the Event, the Republic of Nantucket has done its best to recreate the better ideas of the person of William Walker, renegade Coast Guard officer, who is busy building an empire for himself based on conquest by technology. When Walker reaches Greece and recruits several of their greater kinglets to his cause, the people of Nantucket have no choice. If they are to save the primitive world from being plunged into bloodshed on a twentieth-century scale, they must defeat Walker at his own game: war.